In motor vehicle automatic transmissions known from practice, shifting elements in the form of wet-operating disk clutches or disk brakes are used to transfer torque between a transmission input and a transmission output. In this context, force is transferred by frictional means, by the compression of disk sets of the shifting elements. The pressing force required in each case for this compression of the disk sets is usually produced by hydraulically actuated clutch pistons, which are actuated by means of pressure control valves and clutch valves. Such clutch valves, also known as pressure-reduction valves, are appropriately actuated either directly by a proportional magnet, or by further pressure-limiting valves by means of which a pilot control pressure is adjusted as a function of a control current.
In both methods for actuating the pressure control valves, in each case a magnetic force proportional to the control current is produced, depending on which the hydraulic pressure-reduction valves or clutch valves are actuated. The working pressures of the clutch valves are determined in each case by the equilibrium condition between the control-current-proportional magnetic force or actuating force and the return or reaction force of a clutch valve.
Particularly, pressure control valves actuated by pilot pressures are often made with two poppet valves arranged in a hydraulic semi-bridge circuit, and are known as so-termed closed-end pressure regulators. In their end positions such closed-end pressure regulators are characterized by low leakage since the valve seats of the poppet valves are closed in alternation in the end positions. Thus, despite the large number of shifting elements in an automatic transmission that have to be controlled hydraulically, the hydraulic fluid volume required by a hydraulic system of a motor vehicle automatic transmission can be restricted to a minimum and a hydraulic pump of correspondingly small size can be used.
To reduce the effects of pressure fluctuations in the area between the valve seats of a closed-end pressure regulator, a flow guide element in that area and an additional, external damping element are usually provided. The flow guide element deflects the hydraulic fluid flowing between the two poppet valves in some sections away from the direct flow path, in order to avoid Venturi effects between the poppet valves and the connection to the consumer, but this defined flow guiding increases the hydraulic resistance to an undesired extent and has an adverse effect on the valve dynamics.
To dampen the pressure fluctuations, the damping element comprises a piston element that can be displaced as a function of the pressure present at the time in the last-mentioned area of a closed-end pressure regulator. The piston element has an inherently stable structure under pressure and moves longitudinally in a cylinder housing against a spring device, so that pressure peaks in the closed-end pressure regulator are automatically reduced as a function of the piston element's position, which varies according to the pressure.
In addition or alternatively to this, the piston element of the damping device can be made elastically deformable as a function of pressure so that it is deformed to a greater or lesser extent as a function of the pressure prevailing in the closed-end pressure regulator, in such manner that pressure peaks in the closed-end pressure regulator are in each case reduced to the desired extent by a pressure-dependent deformation and/or as a function of the pressure-dependently varying position of the piston element.
However, closed-end pressure regulators made with damping elements are disadvantageously characterized by high design complexity so that, compared with pressure regulators made without a damping element, their production costs increase to an undesired extent.
In the design of the closed-end pressure regulators described above, the requirements for good control dynamics at the same time as low leakage constitute conflicting demands which can only be resolved by the acceptance of compromises. The geometrical design of the first poppet valve made as a ball-seat valve determines the maximum leakage or the maximum volume flow that can be passed through the closed-end pressure regulator if the second poppet valve, which is often made as a cone-seat or flat-seat valve, is made with essentially larger dimensions. The valve body of the first poppet valve, preferably formed as a ball, is actuated by a push-rod or valve tappet, which after the opening of the first poppet valve, clears the open cross-section of the inlet geometry of the closed-end pressure regulator regardless of the slide's position between the throttle or diaphragm of the first poppet valve and the push-rod.
At low oil temperatures, because of the higher viscosity of the hydraulic fluid, the above-described closed-end pressure regulators have a greatly reduced inlet volume flow, which adversely affects the valve dynamics in particular of pilot-pressure-controlled clutch valves. However, compensation of this temperature-dependent valve dynamics impairment by a larger inlet geometry of the closed-end pressure regulator is not, or is only partially expedient since then, at high hydraulic fluid temperatures, the closed-end pressure regulators have an undesirably large leakage volume flow.